A typical prior art head and disk system 10 is illustrated in block form in FIG. 1. In operation the magnetic transducer 20 is supported by the suspension 13 as it flies above the disk 16. The magnetic transducer 20, usually called a “head” or “slider,” is composed of elements that perform the task of writing magnetic transitions (the write head 23) in the magnetic medium included in the thin films 21 and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths (leads) 14 which are attached to or embedded in the suspension 13. The magnetic transducer 20 is positioned over points at varying radial distances from the center of the disk 16 to read and write circular tracks (not shown). The disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded.
The conventional disk 16 includes substrate 26 of glass or AlMg with an electroless coating of NiP that has been highly polished. The thin films 21 on the disk 16 typically include a chromium or chromium alloy underlayer and at least one ferromagnetic layer based on various alloys of cobalt, platinum and chromium. Additional elements such as tantalum and boron are often used in the magnetic alloy. A protective overcoat layer is used to improve wearability and corrosion resistance. Various seed layers, multiple underlayers and multilayered magnetic films have all been described in the prior art. Laminated magnetic films include multiple ferromagnetic layers that are substantially decoupled. Seed layers are used with nonmetallic substrate materials such as glass. Typically the seed layer is a relatively thin crystalline film which is the first layer deposited on the substrate. Materials proposed for use as seed layers include chromium, titanium, tantalum, MgO, tungsten, CrTi, FeAl, NiAl and RuAl. The use of pre-seed layers 31 is relatively recent practice. The pre-seed layer is a non-crystalline thin film which provides a base for growing the subsequent crystalline films that is superior to the substrate for this purpose.
FIG. 2 illustrates a prior art layer structure 21 of a thin film magnetic disk 16 in which the layer stack according to the invention can be used. The substrate 26 is commonly AlMg/NiP or glass. The layers under the underlayer 33 may be any of several combinations of seed layers 32 and pre-seed layers 31 according to the prior art. The layer structure shown in FIG. 2 can be used with a variety of magnetic layer stacks 34. The magnetic layer stack 34 is composed of a plurality of layers in the prior art including laminated and AF-coupled forms.
Many approaches have been described to improve the media signal to noise ratio (SNR) in magnetic recording media. These techniques often require changing the composition of the magnetic alloy or underlayer, or manipulating the process conditions to achieve the desired microstructure. Another well-established technique for improving media SNR in longitudinal media is by laminating two or more magnetic media layers separated by non-magnetic interlayers. Laminated structures are thought to work best if the two magnetic layers are magnetically uncorrelated and act as independent noise sources. If this holds for two magnetic layers, a 3-dB gain in SNR is expected for the laminated structure as compared to the signal film. This improvement has been described in detail for longitudinal AFC media in U.S. Pat. No. 6,372,330 to Do, et al. This improvement in SNR was achieved in these films without a degradation of other recording performance parameters. The use of lamination for noise reduction has been extensively studied to find favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer thicknesses, from a few angstroms upward, that result in the best decoupling of the magnetic layers and the lowest media noise.
However, the applicability of laminated media in longitudinal recording is limited by thermal stability concerns. As the densities of magnetic storage increase, Mrt (the product of the remanent magnetization and the medium thickness) has decreased and the coercive fields Hc have increased. To achieve this reduction in Mrt, the thickness t can be reduced, but only to a limit. Magnetic media often exhibit (i) decreasing coercive fields and (ii) increasing magnetic decay with decreasing film thickness. These phenomena have been attributed to thermal activation of small magnetic grains or small regions of magnetization (the superparamagnetic effect). The stability of the magnetic media is proportional to KuV, where Ku is the magnetic anisotropy constant of the media and V is the volume of the magnetic grain. As the media thickness is decreased, V also decreases. If the film thickness is too thin, the stored magnetic information is no longer stable in normal hard-drive operating conditions. In conventional laminated media, these problems are exacerbated. For a given Mrt of the composite film structure, each layer will have half the Mrt and, therefore, the onset of the superparamagnetic effect occur for larger Mrt values. Laminated AFC improves this situation but is still limited by thermal stability concerns.
One approach to the solution of this problem is to use a higher anisotropy material, i.e. one with a higher Ku. However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too great to be written by a practical write head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (S0NR) of the magnetic layer.
In U.S. Pat. No. 6,280,813 to Carey, et al. a layer structure is described that includes at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic coupling/spacer film. Antiferromagnetic coupling (AFC) maintains stability of the media with reductions in Mrt. In general, the exchange coupling oscillates from ferromagnetic to antiferromagnetic with increasing coupling/spacer film thickness and that the preferred 6 Angstrom thickness of the ruthenium coupling/spacer layer was selected because it corresponds to the first antiferromagnetic peak in the oscillation for the particular thin film structure. Materials that are appropriate for use as the nonferromagnetic coupling/spacer films include ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium (Ir), copper (Cu), and their alloys. Because the magnetic moments of the two antiferromagnetically coupled films are oriented antiparallel, the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. An embodiment of the structure includes two ferromagnetic CoPtCrB films, separated by a Ru spacer film having a thickness selected to maximize the antiferromagnetic exchange coupling between the two CoPtCrB films. The top ferromagnetic layer is designed to have a greater Mrt than the bottom ferromagnetic layer, so that the net moment in zero applied magnetic field is low, but nonzero. The Carey '813 patent also states that the antiferromagnetic coupling is enhanced by a thin (5 Angstroms) ferromagnetic cobalt interface layer added between the coupling/spacer layer and the top and/or bottom ferromagnetic layers. The patent mentions, but does not elaborate on the use CoCr interface layers. FIG. 3 is an illustration of a magnetic layer stack 34 for a magnetic thin film disk according to the prior art using an exchange enhancing layer 38 under the top magnetic layer 36 in an AF-coupled magnetic structure. The lower magnetic layer 41 is deposited first and serves the role of the slave layer in the AF-coupled structure. The spacer 39 is selected to achieve antiferromagnetic (AF) coupling between the lower magnetic layer and the magnetic layers above it.
In U.S. Pat. No. 6,567,236 to Doerner, et al. (May 20, 2003) an antiferromagnetically coupled layer structure is described for magnetic recording wherein the top ferromagnetic structure is a bilayer structure including a relatively thin first sublayer of ferromagnetic material in contact with the coupling/spacer layer. The first sublayer has a higher magnetic moment than the second sublayer. The second sublayer has a lower magnetic moment and is much thicker than the first sublayer with a composition and thickness selected to provide the Mrt, when combined with the first sublayer, that is needed for the overall magnetic structure. A preferred embodiment of a layer structure according to the patent is a pre-seed layer of CrTi; a seed layer of RuAl; an underlayer of CrTi; a bottom ferromagnetic layer of CoCr; an AFC coupling/spacer layer of Ru; and a top ferromagnetic structure including: a thin first sublayer of CoCr, CoCrB or CoPtCrB, and a thicker second sublayer of material of CoPtCrB with a lower moment than the first sublayer.
Published US patent application 2002/0098390 by H. V. Do, et al., describes a laminated medium for horizontal magnetic recording that includes an antiferromagnetically coupled (AFC) magnetic layer structure and a conventional single magnetic layer. The AFC magnetic layer structure has a net remanent magnetization-thickness product (Mrt) which is the difference in the Mrt values of its two ferromagnetic films. The type of ferromagnetic material and the thickness values of the ferromagnetic films are chosen so that the net moment in zero applied field will be low, but nonzero. The Mrt for the media is given by the sum of the Mrt of the upper magnetic layer and the Mrt of the AF-coupled layer stack. This allows control of the Mrt independently from either Mr or t. Alternatively, the magnetization (the magnetic moment per unit volume of material) of the two ferromagnetic films may be made different by using different ferromagnetic materials for the two. In a laminated medium each of the magnetic layers contributes to the readback signal; therefore, the net magnetic moment of the AFC layer stack must be non-zero. The nonferromagnetic spacer layer between the AFC layer and the single ferromagnetic layer has a composition and thickness to prevent substantial antiferromagnetic exchange coupling. The laminated medium has improved thermal stability from the antiferromagnetic coupling and reduced intrinsic media noise from the lamination.